The present invention relates to a nonresonant type knock sensor that detects the occurrence of knocking in an internal combustion engine.
A knock sensor with a piezoelectric element is commonly used in an ignition control system of an internal combustion engine so as to detect the occurrence of knocking in the engine and thereby allow the control system to provide optimal ignition timing for the engine. There are two types of knock sensors: a resonant type and a nonresonant type. In the case of the nonresonant type knock sensor, the piezoelectric element receives a mechanical load due to engine vibrations caused by the knocking, converts the mechanical load into an electrical signal and outputs the electrical signal to the control system via a band-pass filter so that the control system reads the signal output in a frequency band corresponding to the knocking vibrations to find the occurrence of knocking in the engine.
Recently, there have been strict environmental regulations. When the piezoelectric element is made free from lead so as to be compliant with such strict environmental regulations, there is a possibility that the signal outputted from the lead-free piezoelectric element is so weak that the control system cannot determine whether the knocking is actually occurring in the engine. In order to avoid such a possibility, it is desired to improve the signal output characteristic of the nonresonant type knock sensor.
In consideration of the fact that the intensity of the output signal from the piezoelectric element depends on the mechanical load applied to the piezoelectric element, one conceivable way to improve the signal output characteristic of the sensor would be to increase the size of any part or portion of the sensor that weights down the piezoelectric element (such as a weighting member or resin-molded sensor casing) so as to add to its weight and thereby increase the mechanical load on the piezoelectric element as disclosed in Japanese Laid-Open Patent Publication No. 2-173530. However, this results in upsizing of the sensor. As there is only a limited space for mounting the knock sensor in the engine, it is difficult to improve the signal output characteristic of the sensor to a sufficient degree in the above-mentioned way.
The present invention has been made allowing for the above-mentioned circumstances, and an object of the present invention is to provide a nonresonant type knock sensor that has an increased mechanical load on its piezoelectric element without upsizing of the sensor for improvement in signal output characteristic.
According to a first aspect of the invention, there is provided a knock sensor, comprising: a sensor body having: a metallic shell including a cylindrical portion and a flange portion formed at an end of the cylindrical portion; an annular piezoelectric element fitted around the cylindrical portion; and an annular weighting member fitted around the cylindrical portion to hold the piezoelectric element between the weighting member and the flange portion; and a resin-molded sensor casing arranged circumferentially around the sensor body, wherein the resin-molded sensor casing includes a weighting portion located nearer to the weighting member than to the piezoelectric element with respect to an axial direction of the cylindrical portion, and at least the weighting portion of the resin-molded sensor casing is made of a resin containing at least one of metal powder and metal oxide powder and has a density of 2.0 g/cm3 or higher.
According to a second aspect of the invention, there is provided a knock sensor, comprising: a metallic shell including a cylindrical portion and a flange portion formed at an end of the cylindrical portion; an annular piezoelectric element fitted around the cylindrical portion; and an annular weighting member fitted around the cylindrical portion to hold the piezoelectric element between the weighting member and the flange portion, wherein at least the flange portion of the metallic shell is made of a material having a lower specific gravity than that of iron.
According to a third aspect of the invention, there is provided a knock sensor, comprising: a metallic shell including a cylindrical portion and a flange portion formed at an end of the cylindrical portion; an annular piezoelectric element fitted around the cylindrical portion; and an annular weighting member fitted around the cylindrical portion to hold the piezoelectric element between the weighting member and the flange portion, wherein the flange portion has at least one cut formed therein to reduce the weight of the flange portion.